Rouge black hole in the Milky Way
Credit: David Aguilar (CfA)

Recoiling Black Holes

When two black holes merge, they may release gravitational waves
preferably in one direction causing them to be 'kicked' in the
opposite direction. Because the velocity of these kicked black holes
can be large enough to eject them from galaxies, there has been a
surge in research on trying to find such recoiling black holes.
Avi Loeb and I showed that during the formation of the Milky Way,
hundreds of black holes may have been kicked into the halo of the
galaxy (yikes!). More importantly, however,
is that we showed that these black holes would be surrounded by a
cluster of stars that can show us where these black holes are today,
and how massive the black hole was when it was kicked. The discovery
of even one of these objects would help provide us with a window into
how supermassive black holes formed in the early universe.

More recently, I have refined our calculations of the evolution of these clusters. I found that large angle scatterings and tidal disruptions remove or destroy many, but not all stars in the cluster. I used our results to perform a systematic search for candidates in the Sloan Digital Sky Survey. I found no spectroscopic candidates, in agreement with our expectations from the completeness of the survey. Using generic photometric models of present day clusters I identified ~100 recoiling cluster candidates. Follow-up spectroscopy would be able to determine the nature of these candidates.